Field of the Invention
The present invention relates to a wire electric discharge machine with a machining fluid supply device.
Description of the Related Art
Conventionally, in an immersion-type wire electric discharge machine, a sealing portion for leakage prevention is provided on a sidewall portion of a machining tank penetrated by an arm through which a wire electrode is passed. Generally, in a wire electric discharge machine such as a wire-cut electric discharge machine, a machining fluid is supplied at a constant rate to the sealing portion of the machining tank (see Japanese Patent Application Laid-Open No. 2006-297570).
Japanese Patent Application Laid-Open No. 8-1443 discloses a method in which a flow sensor disposed in a fluid supply passage is used to detect the flow rate of a machining fluid supplied from a clean water tank to an electric discharge machining part, and the speed of a filter pump is inverter-controlled so that the flow rate of the machining fluid supplied from a dirty tank to the clean water tank is slightly higher than that of the machining fluid supplied to the machining tank. In this method, the machining fluid is supplied only from the machining fluid supply passage that is used during electric discharge machining.
Japanese Patent Application Laid-Open No. 2005-46984 discloses a method in which fluid surface level detecting means attached to a clean water tank is used to obtain the discharge rate of a filter pump based on a difference in height with respect to a reference fluid surface position, in order to prevent early clogging of a filter, and the discharge rate of the filter pump is controlled so that the fluid surface position is restored to the reference position. However, this document does not describe how early clogging of the filter is prevented by reducing the flow rate of a machining fluid supplied from the clean water tank to a machining tank to reduce the flow rate of the machining fluid flowing into the filter.
Japanese Patent Application Laid-Open No. 9-216130 discloses a technique in which a flow regulating valve is provided for adjusting the flow rate of a machining fluid supplied to a machining tank attached to a machining fluid supply pipe and the open/close operation of the flow regulating valve is controlled in response to information from a machining tank fluid surface sensor.
Japanese Patent Application Laid-Open No. 4-322914 discloses a technique in which a concentration of sludge in a machining tank is detected and if the detected concentration is higher than a preset value, then a circulating liquid and a machining fluid are increased in amount to reduce the sludge concentration.
An example of a machining fluid supply device of a conventional wire electric discharge machine will now be described with reference to FIG. 3.
Conventionally, in an immersion-type wire electric discharge machine, a sealing portion 4 for leakage prevention is provided on a sidewall portion of a machining tank 3 penetrated by an arm through which a wire electrode is passed. A machining fluid in the machining tank 3 is discharged into a dirty tank 2 through a drain portion 15. The machining fluid discharged from the machining tank 3 into the dirty tank 2 contains machining chips of a workpiece and the wire electrode generated by electric discharge machining. The machining fluid is supplied from a clean water tank 1 to the machining tank 3. The machining fluid to be supplied to the clean water tank 1 is supplied from the dirty tank 2 through a filtration filter 5. The filtration filter 5 removes the machining chips from the machining fluid supplied from the dirty tank 2 to the clean water tank 1. A machining fluid pump 7 draws up the machining fluid from the clean water tank 1 and supplies it to nozzles (not shown) of the machining tank 3. A controller 6 drivingly controls the machining fluid pump 7 through an inverter 11b. 
In controlling a filter pump 8 through an inverter 11a in association with the surface position of the machining fluid in the clean water tank 1 and the amount of the machining fluid used in the tank 1, the machining fluid flows excessively to the sealing portion 4 so that the fluid level of the clean water tank 1 is easily reduced if the supply of the machining fluid to the sealing portion 4 by a feed water pump 9 is constant. Thus, the machining fluid flows at too high a flow rate from the dirty tank 2 to the clean water tank 1 through the filtration filter 5.
Since a pressure applied to the filtration filter 5 is proportional to the square of the filter flow rate, it easily increases if the supply of the machining fluid to the sealing portion 4 is constant. Thus, there is a problem that the service life of the filter 5 is reduced.